Buyer beware, as with all online auctions. Gunbroker or whatever auction site is not the seller, so if there is an issue, its between you and them. I would say most are honest, but a bit over optimistic in terms of condition. Their idea of 98% gun is certainly not my idea of the same condition. I also see shill bidding too, here and there, but as long as you place limits on what you are willing to spend and be patient, it doesn't matter.
Its hard to judge quality without good pics, so know that if you are bidding on something that has crappy pics, you just got yourself into a gamble. It might be as described, it might not. Also, if you want to sell something and get top dollar for it, take good pics. That means in focus and covering all the details. Close ups. I'll bid more on something that I can see clearly, and lower my bids for something that isn't so well documented.
Pay attention to return policies. I've had one person who didn't honor his return policy for a misrepresented gun, so I ended up eating it. That hurts both pocketbook and pride, but I decided there were no other real avenues available for recourse. Above all, make sure there is a return policy. I won't buy from anyone who doesn't have an inspection period. I want to see it to make sure I'm not getting screwed.
You gotta look at the feedback too, it'll tell you a lot about the person selling or buying. All in all, i've bought and sold around thirty guns off of auction sites and have had two really bad experiences with sellers. Not that good of odds, really. Be sure to really inspect guns on the day you get them, and don't let them sit at the dealers beyond the inspection period. And most of all, don't feel bad or be scared about sending things back if they don't meet the description or are misrepresented. You'll be out the shipping, but that's better than a POS that you will be stuck with otherwise.
Generally, the two most popular auction sites are gunbroker.com and auctionarms.com. gunbroker is larger, with more stuff, but there is a lot on auctionarms too. I use both.
There are deals to be had, occasionally, but not that often. Be patient, and don't get the idea you have to have any one gun. That is how you get burned the quickest. There will be another come up for sale eventually, so wait.
That's my .02 cents.